Hundreds of council tax discounts could be removed after auditors uncovered “pervasive” errors with the SIC”s system.
The SIC currently provides discounts or exemptions to almost half of the 11,500 homes throughout the isles.
However a recent review of council tax found problems with almost half of the 60 cases investigated.
The review found discounts or exemptions had either been incorrect, not due, or issued with insufficient evidence to confirm their suitability.
Internal auditor Duncan Black said the errors were “quite pervasive” and made four high priority recommendations, requiring “urgent attention”.
Addressing today’s audit committee, Mr Black said it would be “quite a lengthy piece of work” for the council to resolve.
He said the changes would mean more revenue for the council – although the total value had yet to be quantified.
The auditor suggested the council should be “quite bullish” in chasing tax that had been withheld fraudulently.
He also called on the council to carry out annual or two-yearly reviews of all discounts and exemptions to ensure they remained valid.
People entitled to exemptions or discounts include students, disabled people, people in long-term hospital care, single people, and Ukrainian refugees.
However, Mr Black said the council’s website information was not complete as not all discount and exemption types were covered and some was out of date.
He also noted the council had no policy on whether to make use of powers to charge double council tax on long-term empty homes or second homes.
Senior management have accepted the recommendations and said reviews would be carried out.
Officers will report to councillors regarding the possible double charges.
Committee chairman Allison Duncan said the number of recommendations seemed quite high and called for officials to progress them with “some urgency”.